[PATCH 2/3] arm64: mm: Set PSTATE.PAN from the cpu_enable_pan() call

[james.morse@arm.com (James Morse)]

Commit 338d4f49d6f7 ("arm64: kernel: Add support for Privileged Access
Never") enabled PAN by enabling the 'SPAN' feature-bit in SCTLR_EL1.
This means the PSTATE.PAN bit won't be set until the next return to the
kernel from userspace. On a preemptible kernel we may schedule work that
accesses userspace on a CPU before it has done this.

Now that cpufeature enable() calls are scheduled via stop_machine(), we
can set PSTATE.PAN from the cpu_enable_pan() call.

Add WARN_ON_ONCE(in_interrupt()) to check the PSTATE value we updated
is not immediately discarded.

Reported-by: Tony Thompson <anthony.thompson@arm.com>
Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: James Morse <james.morse@arm.com>

---
This patch depends on 'arm64: cpufeature: Schedule enable() calls instead
of calling them via IPI', which doesn't apply to linux-stable versions
before v4.8.

 arch/arm64/mm/fault.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 3e9ff9b0c78d..f942ab6cc206 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -29,7 +29,9 @@
 #include <linux/sched.h>
 #include <linux/highmem.h>
 #include <linux/perf_event.h>
+#include <linux/preempt.h>
 
+#include <asm/bug.h>
 #include <asm/cpufeature.h>
 #include <asm/exception.h>
 #include <asm/debug-monitors.h>
@@ -672,7 +674,14 @@ NOKPROBE_SYMBOL(do_debug_exception);
 #ifdef CONFIG_ARM64_PAN
 int cpu_enable_pan(void *__unused)
 {
+	/*
+	 * We modify PSTATE. This won't work from irq context as the PSTATE
+	 * is discared once we return from the exception.
+	 */
+	WARN_ON_ONCE(in_interrupt());
+
 	config_sctlr_el1(SCTLR_EL1_SPAN, 0);
+	asm(SET_PSTATE_PAN(1));
 	return 0;
 }
 #endif /* CONFIG_ARM64_PAN */
-- 
2.8.0.rc3